Recently, demand for secondary batteries as an energy source has been significantly increased as portable small electronic devices have been commercialized while realizing the miniaturization and weight reductions of electronic equipment. A lithium secondary battery exhibits two or more times higher discharge voltage and higher energy density than a typical battery using an alkaline aqueous solution by using an organic electrolyte solution.
A lithium secondary battery is prepared by using materials capable of intercalating and deintercalating lithium ions as an anode and a cathode, and charging an electrolyte solution between the cathode and the anode. Also, the lithium secondary battery generates electrical energy by oxidation and reduction reactions when lithium ions are intercalated and deintercalated in the cathode and the anode.
Lithium secondary batteries may be classified into lithium ion batteries using a liquid electrolyte solution and lithium-ion polymer batteries using a polymer solid electrolyte solution depending on the type of electrolyte solution. Also, depending on the type of the polymer solid electrolyte solution, the lithium-ion polymer batteries may be classified into full-solid type lithium ion polymer batteries, in which an electrolyte solution is not contained at all, and lithium-ion polymer batteries that use a gel-type polymer electrolyte solution containing an electrolyte solution.
With respect to the lithium ion batteries using a liquid electrolyte solution, a cylindrical or prismatic metal can container is generally used and sealed by welding. With respect to a prismatic lithium secondary battery, it is advantageous in protecting an electrode assembly from external impact and facilitating a liquid injection process. However, since the shape of the prismatic lithium secondary battery is fixed, it is difficult to reduce the volume thereof. Thus, with respect to electrical products using the prismatic lithium secondary battery as a power source, there are limitations in their design. Also, in terms of safety, since an effect of venting gas or liquid is not significant, internal heat and gas are accumulated. Thus, the risk of explosion may be high, and the time required to cause cell degradation may be short due to overheating because the internal heat is not effectively released.
In order to improve such limitations, pouch-type secondary batteries have recently been developed, in which the pouch-type secondary batteries are prepared by putting an electrode assembly, in which a cathode, an anode, and a separator are stacked and wound, in a pouch-type case formed of a film, injecting an electrolyte solution, and then sealing the pouch-type case.
As illustrated in FIG. 1, the pouch-type case is sequentially composed of an internal layer a acting as a sealant due to its sequential heat sealability, a metal layer b acting as a moisture and oxygen barrier layer while maintaining mechanical strength, and an external layer c acting as a protective layer. In this case, the internal layer a is formed in a multilayer structure in which a casted polypropylene (CPP) layer 11 commonly used as a polyolefin-based resin layer and a PPa layer 13 are stacked, the metal layer b is formed of a aluminum (Al) layer 15, and the external layer c is formed in a multilayer structure in which a polyethylene terephthalate (PET) layer 17 and a nylon layer 19 are stacked.
With respect to the pouch-type secondary battery, it has advantages in that there are no limitations in shape and size, assembly by thermal fusion is easy, and it has high safety because the effect of venting gas or liquid is facilitated when abnormal behaviors occur. Thus, the pouch-type secondary battery is particularly suitable for the preparation of a thin cell.
However, since the pouch-type secondary battery, different from the prismatic secondary battery, uses a soft pouch as a case, the case of the pouch-type secondary battery may have low physical and mechanical strength and may have low reliability of sealing.
For example, lithium hexafluorophosphate (LiPF6) is included in the electrolyte solution used in the pouch-type secondary battery, wherein since the lithium hexafluorophosphate is decomposed into lithium (Li) and PF6 during charge and discharge process to provide lithium ions to an electrolyte, it is used to increase the diffusion rate of lithium ions. However, since the lithium hexafluorophosphate has very good hydrophilicity, the lithium hexafluorophosphate reacts with moisture (H2O) having a relative humidity of a few percent (%) that is included in air to form lithium hydroxide (LiOH) as well as hydrofluoric acid (HF) gas due to a reaction of H+ with a single fluorine (F) atom of PF6. The HF gas may increase the thickness of the pouch and may further cause the explosion.
In addition, since the metal layer b is exposed while the internal resin layer a is damaged due to internal stress during case forming to react with ions 23 of lithium salt that are dissolved in the electrolyte solution, a lithium-aluminum alloy 25 is formed on the surface of the metal layer b. The lithium-aluminum alloy 25 thus formed may further damage the pouch case while reacting with infiltrated moisture 27 to form HF gas and micropores. As a result, since HF gas increases while moisture penetration in the damaged pouch is facilitated, the dissolution of the Li—Al alloy becomes more intensified under strong acidic conditions. Therefore, the size of the pores is gradually increased 29. Eventually, this may cause the corrosion of the pouch case to result in the leakage of the electrolyte solution (see FIG. 2).